This section provides background information related to the present disclosure which is not necessarily prior art.
Many machines (e.g., electric motors) include a rotating shaft to which torque is applied. In an electric motor, for instance, the active elements produce interacting magnetic fields that turn the machine's rotor and, thus, the shaft of the motor.
Such machines can also experience cogging. For instance, in the electric motor, passive interaction between the magnetic elements of the stator and rotor can cause cogging that sequentially and cyclically applies positive torque (aiding rotation of the shaft) and negative torque (hindering rotation of the shaft). The cogging torque can cause undesirable vibrations of the motor and can be especially noticeable at low rotational speeds.
Other machines, such as a cam system, experience cogging loads as well. For instance, some valve systems rotate a shaft to cam a valve open against the biasing force of a spring, wherein further rotation of the shaft allows the spring to bias the valve back closed. Thus, the spring supplies a negative cogging torque to the shaft (i.e., in a direction opposing rotation of the shaft) as the valve opens, and the spring supplies a positive cogging torque to the shaft (i.e., in the same direction as rotation of the shaft) as the valve closes. This cogging torque repeats cyclically as the shaft rotates about its axis.
Similarly, in an internal combustion engine, the crank shaft rotates to actuate a piston in an engine cylinder. As the volume in the cylinder is reduced and pressure increases therein (i.e., during the compression cycle), the crank shaft can experience a resultant negative cogging torque. Conversely, as the volume is increased and pressure decreases therein, the crank shaft can experience a resultant positive cogging torque. Cogging torque can be especially noticeable if combustion is not actually occurring (e.g., during engine startup, where one or more cylinders are deactivated, etc.).